Turbine PilotTurbine Pilot

Breaking the ChainBreaking the Chain

April 1, 2007

Enhanced ground proximity warning systems come of age Late in the morning of December 1, 1974, TWA Flight 514 hit the side of a mountain about 25 miles west of what was then known as Dulles International Airport in Washington, D.C. None of the 85 passengers and seven crewmembers survived the accident, which occurred in instrument meteorological conditions (IMC) after the aircraft descended prematurely while the crew prepared to conduct the VOR/DME approach to Runway 12 at Dulles.

Enhanced ground proximity warning systems come of age

Late in the morning of December 1, 1974, TWA Flight 514 hit the side of a mountain about 25 miles west of what was then known as Dulles International Airport in Washington, D.C. None of the 85 passengers and seven crewmembers survived the accident, which occurred in instrument meteorological conditions (IMC) after the aircraft descended prematurely while the crew prepared to conduct the VOR/DME approach to Runway 12 at Dulles. (See " Safety Pilot Landmark Accidents: Cleared for the Approach," June 1998 Pilot.) The aircraft was not equipped with a ground proximity warning system, or GPWS.

At the time, TWA Flight 514 was only the latest in a long string of similar controlled flight into terrain, or CFIT, accidents that seemed to occur with regularity to airline flights throughout the world. A CFIT accident is one in which an airworthy aircraft under the control of the flight crew is unintentionally flown into terrain, obstacles, or water. CFIT can occur during any airborne phase of flight, but is most common during the approach and landing.

CFIT is among the most perplexing of accident types, since seemingly all it takes for a crew to avoid the accident in the first place is awareness of the aircraft's location in relation to threatening terrain or obstacles. Yet time after time, professional flight crews fail to maintain situational awareness. The ensuing accident investigations often finger fatigue, confusion, and faulty crew decision making as common culprits in the chain of events leading to a CFIT accident. Advances in aviation safety have long been paid for in both lives and money, and CFIT prevention is no exception. By the late 1960s, a young engineer named Don Bateman had begun working on a device that took information from an aircraft's radio altimeter and combined it with computational logic to provide terrain warnings to pilots under certain conditions. By the beginning of the 1970s a few airlines had begun voluntarily installing GPWS in their aircraft, but most hadn't. TWA Flight 514 finally spurred the FAA to require U.S. Part 121 air carriers to install GPWS on board their large jet and turboprop aircraft.

Better than nothing

The first GPWS models depended solely on the aircraft's downward-directed radar altimeter beam to detect terrain. Thus the system was reasonably good at warning crews of gradually rising terrain, but not much help in warning of sheer terrain (such as a cliff face or steep mountain ridge) in front of a fast-moving aircraft. Warning times might range from just a few seconds to perhaps 15 to 30 seconds, depending on the kind of terrain toward which the aircraft was flying — not great, but certainly better than nothing. Unfortunately these early versions of GPWS were plagued with numerous false warnings. As a result, flight crews tended to ignore them altogether or waste precious time deciding whether the warning was valid. The result was that although some CFIT accidents were avoided, too many others continued to occur.

In fact, CFIT still makes up the single largest accident category for commercial jet aircraft worldwide. According to a Boeing study, from 1987 through 2005 there were 57 fatal CFIT accidents involving the western-built commercial jet fleet (defined as jet airplanes heavier than 60,000 pounds). These accidents claimed 3,735 lives and dwarfed all other accident categories in terms of both the number of accidents and resulting fatalities.

Yet the statistics mask a bright spot in the fight against CFIT. To dramatically illustrate it, let's fast-forward three decades from TWA Flight 514 to another airline crew shooting a nonprecision VOR approach, this one operating a de Havilland Dash 8 in night IMC to Hayden, Colorado, in 2004. Just as with TWA 514, this incident involved the aircraft's premature descent below a safe altitude. As told by one of the crew in a NASA Aviation Safety Reporting System (ASRS) report,"...We were cleared to intercept the HDN 105-degree radial, maintain 13,000 feet until established, cleared for the VOR DME B approach to Hayden. We intercepted the 105-degree radial and called established at 28 DME. We began a descent to 10,000 feet as we thought was depicted on the approach plate. We received a GPWS terrain alert at 11,500 feet and initiated a climb until the warning ceased. Further study of the approach plate showed [that] the 10,000 feet is for a DME arc and the approach didn't begin until 17 DME."

The difference in outcomes was partly the result of the latest iteration of terrain warning technology, known as enhanced GPWS. EGPWS (also referred to as a terrain awareness warning system, or TAWS) is a far more sophisticated kind of warning system that combines a worldwide terrain database with the positioning accuracy of GPS. Since it always knows where the airplane is in relation to terrain, EGPWS can provide pilots with much more warning time. Its warnings are more instantly believable too, since pilots not only hear the warning but also in many aircraft installations, see the terrain depicted on their aircraft's map display in relation to their current position.

The crew of a Boeing 737-400 on a nighttime visual approach to Runway 11 at Tucson, Arizona, in 2002 offered testament to the effectiveness of EGPWS. As described in the ASRS report of the incident, "The EGPWS gave a 'Terrain, Terrain' warning due to the 4,682-foot peak just south of the localizer at 15 miles. Due to a long day, some fatigue, complacency, and a clear night with unlimited visibility, we accepted a visual too far out and began a descent too soon. The good news: technology saved the day!"

It works

EGPWS also happens to be the brainchild of Don Bateman, now chief engineer, Flight Safety Systems for Honeywell. Since its introduction in the mid-1990s, its effectiveness in the battle against CFIT has been nothing short of phenomenal. Bill Reavis, a spokesman for Honeywell, puts it bluntly: "No EGPWS-equipped aircraft has had a CFIT accident, period."

According to Reavis, as of the beginning of 2007 Honeywell had identified more than 50 incidents in which EGPWS is credited with preventing a possible CFIT accident. This figure includes helicopter, turboprop, and jet aircraft operations involving both airline and corporate operators, and not just large jet aircraft. Beginning March 29, 2005, the FAA required that all jet and turboprop aircraft equipped with a total of six seats or more and operated under FAR Part 91 be equipped with TAWS-B. (See " Steering Clear: TAWS Laws," November 2004 Pilot.)

The disconnect between ongoing worldwide CFIT accidents and the complete cessation of such accidents among EGPWS-equipped aircraft raises the question of why more operators around the world haven't yet replaced older-generation GPWS with EGPWS. So far the lion's share of CFIT accident risk reduction in large jet aircraft operations has occurred in North America and Western Europe, where the technology has been most widely embraced by operators and regulators.

To put the improvements in perspective, from 1965 through 1974 large western-built jet aircraft were involved in CFIT accidents in these regions about once in every 830,000 flights. From 1995 through 2002, coinciding with the advent of EGPWS, this figure had improved drastically to once in every 91 million flights. Not all of the improvement is directly attributable to EGPWS, since better training and other initiatives have been undertaken by the aviation industry to try to eliminate CFIT accidents. But its contribution is undeniable.

In contrast, the rest of the world hasn't experienced anything like this rate of improvement. For the same period, from 1965 through 1974, the CFIT accident rate elsewhere around the world was about one CFIT accident per every million flights, improving to only one per every 1.6 million flights in the period from 1995 through 2002. These figures also reflect the reality that poorer regions of the world often have less capable air traffic control systems, fewer resources to spend on training and equipment, and weaker overall aviation infrastructures compared with more highly developed regions.

Multilayered approach

As good as EGPWS is, pilots need to keep in mind that technology alone can't be depended upon to save them from a CFIT accident. Good crew resource management (CRM), adherence to standard operating procedures, and attention to the business of flying are equally important components to preventing an accident. The fatigued crew of an EGPWS-equipped Bombardier CL-65 regional jet on vectors to an ILS approach at University Park Airport in State College, Pennsylvania, discovered this firsthand late one night in 2003.

Following a breakdown in CRM, the first officer (the flying pilot) entered an incorrect altimeter setting that caused the aircraft to level off more than 1,000 feet below its assigned altitude of 4,000 feet following a descent. Meanwhile, the captain had left the frequency to call the company on the number-two VHF radio. Shortly after level off the EGPWS sounded a terrain warning, which the FO confused with the aircraft's low speed warning system alert. By the time the captain returned his attention back to the flight, the radar altimeter indicated the aircraft was just 900 feet above terrain. According to the ASRS report of this incident, "Since the crew thought they were already at 4,000 feet they did nothing. Only after the controller told the crew to climb, in an excited voice, did the FO start a climb." The report goes on to say that the flight came within an estimated 85 feet of striking an antenna.

As this incident shows, keeping CFIT at bay will always require a multi-layered approach involving pilots, controllers, and technology. Meanwhile, EGPWS is proving its mettle as a big part of the equation.

Vincent Czaplyski flies as a Boeing 757/767 captain for a major U.S. airline.